1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to systems and techniques for generating waveforms for code-division multiple access (CDMA) systems.
2. Description of the Related Art
Several digital modulation techniques are known in the art including code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA). The spread spectrum modulation technique of CDMA has significant advantages over other digital modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990 to Gilhousen et al., entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
Typically, a plurality of analog signals from a public switched telephone network (PTSN) are digitized and consolidated into a high rate digital stream. The high rate digital stream is then disassembled into a plurality of packets which are distributed to a plurality of channel elements. The channel elements convolutionally encode the packets and perform numerous additional functions including adding CRC (cyclic redundancy checking) bits, convolutional encoding, power adjustment and orthogonal spreading using Walsh sequences. The outputs of the channel elements and a pilot signal are then typically summed and input to a waveform generator which creates a waveform suitable for transmission.
The method for providing digital wireless communications using CDMA was standardized by the Telecommunications Industry Association (TIA) in TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereafter IS-95). In accordance with this standard, transmission of voice and/or data from a base station over a forward link from a land line to a mobile unit requires the generation of specific waveforms by the CDMA transmission system. U.S. Pat. No. 5,103,459 (""459), issued Apr. 7, 1992 to Gilhousen et al., entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, (assigned to the assignee of the present invention and incorporated by reference herein) discloses and claims an advantageous system and technique for generating such waveforms for use in a CDMA cellular telephone system.
As disclosed by Gilhousen et al., the waveforms are generated through the use of finite impulse response (FIR) filters which band limit the transmit waveform Multiple voice/data channels are typically provided along with sync and paging channels. There may be as many as 64 channels in total. For each channel, two FIR filters are typically required to generate waveforms for the I and Q subchannels required for a quadrature output. Hence, for a typical system, as many as 128 FIR filters may be required, two for each channel.
As FIR filters are costly to implement, a need remains in the art for a system or technique allowing for a more efficient, less costly generation of waveforms for use in CDMA cellular telephone forward links and other applications.
The need in the art is addressed by the finite impulse response filter and filtering method of the present invention. The inventive filter includes a first circuit for providing plural delayed signals in response to an input signal. A second circuit is included for multiplying respective ones of the delayed signals by a corresponding coefficient and providing a respective intermediate signal in response thereto. A third circuit selectively changes the sign of respective ones of a first set of the intermediate output signals to provide a set of component in-phase signals. A fourth circuit selectively changes the sign of respective ones of a second set of the intermediate output signals to provide a set of component quadrature signals. The component in-phase signals are combined to provide an in-phase output signal and the component quadrature signals are combined to provide a quadrature output signal.
In the illustrative implementation, the coefficients are generated in accordance with an industry standard via a storage device such as a shift register. The third and fourth circuits are controlled by a pseudo-noise sequence generator.
The inventive implementation affords a considerable degree of efficiency in design in that in-phase and quadrature filter outputs are generated from a single filter thereby obviating the need for a second filter in accordance with conventional teachings.